1. Field of the Invention
This invention applies to digital communication systems and particularly to detecting the presence of digital data having known characteristics.
2. Description of the Prior Art
U.S. Pat. No. 4,167,700, "Digital Voice Protection System and Method," issued Sept. 11, 1979, to Coe et al., describes a radio communication system that carries voice messages, in either analog or digitally encoded format, and function control tones. The system includes base stations, mobile and portable radios, repeaters, and satellite receivers to provide wide geographic coverage. A feature of the system is automatic control of remotely located equipment, accomplished in part by digital signal detectors that examine the communication channel for the presence of digital data having known characteristics.
U.S. Pat. No. 4,197,502, "Digital Signal Detector," issued Apr. 8, 1980, to Sumner et al., describes a typical detector. The receiving equipment demodulates the waveforms from the channel and couples them to a limiter to create binary digital signals. A "phase lock" detector determines whether the binary signal is synchronous with a predetermined clock rate by attempting to phase lock a local clock to the limiter output. For a predefined time interval, a data transition counter accumulates a net count of transitions that occur close to the clock phase where data transitions are expected minus those that occur close to the opposite phase, where there definitely should be none. The count ignores transitions at other times, which would be produced by noisy or unreliable data.
On the average, the net count from random noise is zero, but there is finite probability to accumulate a count that results in "false detection." Similarly, hard-limited analog voice signals produce random transitions with finite probability of false detection. To prevent "falsing," the detector requires that the count exceed a threshold and uses hysteresis to decrease the threshold after the first detection to prevent intermittent detection. The threshold can also set a minimum frequency for control tones that will be detected. Below a certain frequency, too few transitions accumulate in the count interval, and the detector ignores the tones.
From a count above threshold, the equipment can infer that the received signal is digital data of use to it or a function control tone at a submultiple of the clock rate, both of which are considered "valid" data. Correlation between successive bits can be performed to distinguish between these two possibilities. Counts below threshold imply noise-like, random signals, which could be analog voice, idle channel noise, or data and tones at other clock rates, which are all considered "invalid" data.
Prior art digital signal detectors have several deficiencies. Detection times at high bit-error-rates are highly variable. If the detector fails to recognize valid data during one interval, it will require at least another full interval. Because high sensitivity requires long counting intervals, sensitive detectors burden throughput. Furthermore, the equipment ignores the incoming signal until it has decided that it constitutes valid data. In systems using preamble sequences to synchronize or activate local circuits, ignoring initial data shortens the effective preamble and degrades performance. Previous methods also leave an ambiguity in exact detection time equal to the length of the count interval and require truncating the signal by the full length of the count interval to insert control information into the signal path.